1. Field of the Invention
The present invention relates to a vacuum source device of a vacuum booster which is used for boosting, for example, the brake master cylinder in automobile or the like vehicles.
2. Description of the Prior Art
The vacuum source of conventional vacuum booster for vehicles has a vacuum chamber which is connected through a check valve to a vacuum pick-up port opening to a portion of the intake system downstream of the throttle valve of an internal combustion engine, so as to accumulate the vacuum which serves as the power source for the vacuum booster.
The level of the vacuum established in internal combustion engine varies depending on the type of employed engine and kinds of accessories attached to the engine. For instance, when the engine is 2-cycle engine or an engine equipped with a multiple type carburetor or when the engine incorporates an automatic transmission, the level of the vacuum is as low as -300 to -400 mmHg, while ordinary engine can produce a vacuum which is as high as -500 mmHg or so. When only a low level of vacuum is available such as in a conventional vacuum booster, it is necessary to increase the pressure-receiving area in the booster to obtain the necessary boosting ratio. This, however, requires an increase in the size of the booster as a whole, making it difficult to mount the booster in a limited space in the engine room.